1. Field of Invention
This invention relates to sound absorbing structures. More particularly, this invention relates to structures which may be added to walls or ceilings and which are designed to absorb sound particularly at frequencies of less than about 1000 Hz.
2. Prior Art
The acoustics of a room or other enclosure depend primarily upon the acoustical properties of its walls, floor and ceiling. Depending upon which material or combination of materials is chosen, the sound absorption of a particular room may vary widely. Wooden paneling, for example, when backed by an air space which may be present when paneling is installed over furring strips, is a moderate absorber of low frequency sound but provides little absorption at frequencies above about 1000 Hz. Draperies and curtains moderately absorb medium and higher frequency sounds but absorb little of lower frequency sounds, particularly when they are installed or maintained in close proximity to a rigid wall. Carpeting, in contrast, is relatively effective as an absorber of high frequency sounds but provides little absorption at the lower end of the audible range of acoustic frequencies.
Concrete, masonry, masonry blocks and gypsum boards are frequently employed in modern construction. However, most of these materials are extremely hard and absorb little, if any, sound. Sound damping may be obtained by employing carpeting on floors and by installing porous materials such as acoustical ceiling tiles. However, covering ceilings and floors does not adequately solve all acoustical problems. In fact, even in the presence of carpeting and acoustical ceiling tiles, many sounds will produce ringing or flutter echoes which reflect back and forth between the surfaces of parallel, reflective walls formed of masonry or plaster.
Masonry and other rigid sound absorbing structural elements have been disclosed in patents such as U.S. Pat. No. 2,933,146, in which each masonry block cavity resonates in a Helmholtz manner (see pp. 42-44, Sensations of Tone, Herman Helmholtz) with a slot in the cavity wall. U.S. Pat. No. 4,071,989 also discloses a block-type acoustical resonator but these patents do not provide for continuous panels enclosing a single resonance cavity as disclosed herein. U.S. Pat. No. 2,007,130 describes a sound absorption unit which is formed from terra cotta. The cavity behind longitudinal slits disclosed in this patent is completely filled with a sound absorbing material which eliminates Helmholtz resonance absorption. These units are load bearing elements which are too heavy and too costly for use in normal decorative applications.
The use of curved wall units is also known. See, for example, U.S. Pat. No. 2,913,075. Again, however, the unit described in the patent does not provide the combination of acoustical properties that the applicants' invention does.
U.S. Pat. No. 2,335,728 discloses a floor unit in which the cavity behind the face plate may be open.
U.S. Pat. No. 2,989,136 discloses a sound attenuation mechanism primarily for use with aviation engines. The individual panels, as demonstrated in FIG. 6, require that the opening at 112 be relatively similar in size to the length of the covering bodies 116.
Accordingly, none of these references or patents disclose the use of elongated, thin-walled panels of the type disclosed herein.
Thus it is an object of this invention to provide a sound absorbing wall structure which has enhanced, low frequency, sound absorption.
It is another object of this invention to provide sound absorptive wall and ceiling structures which may be applied to standard structural room walls and ceilings for decorative effect.
It is a further object of this invention to provide panel mounting means which will allow panels to better absorb sound mechanically at the mechanical resonance frequencies of the mounted panels themselves.
Another object of this invention is to provide a system which will absorb sound across much of the audible sound frequency range.
Yet another object of this invention is to provide a sound absorptive wall structure which can easily be installed over existing walls by installers having limited skills.
These and other objectives are obtained by constructing the apparatus of the instant invention.